Some embedded speech coders such as ITU-T G.718 and G.729.1 compliant speech coders have a core code excited linear prediction (CELP) speech codec that operates at a lower bandwidth than the input and output audio bandwidth. For example, G.718 compliant coders use a core CELP codec based on an adaptive multi-rate wideband (AMR-WB) architecture operating at a sample rate of 12.8 kHz. This results in a nominal CELP coded bandwidth of 6.4 kHz. Coding of bandwidths from 6.4 kHz to 7 kHz for wideband signals and bandwidths from 6.4 kHz to 14 kHz for super-wideband signals must therefore be addressed separately.
One method to address the coding of bands beyond the CELP core cut-off frequency is to compute a difference between the spectrum of the original signal and that of the CELP core and to code this difference signal in the spectral domain, usually employing the Modified Discrete Cosine Transform (MDCT). This method has the disadvantage that the CELP encoded signal must be decoded at the encoder and then windowed and analyzed in order to derive the difference signal, as described more fully in ITU-T Recommendation G.729.1, Amendment 6 and in ITU-T Recommendation G.718 Main Body and Amendment 2. However this often leads to long algorithmic delays since the CELP encoding delays are sequential with the MDCT analysis delays. In the example, above, the algorithmic delay is approximately 26-30 ms for the CELP part plus approximately 10-20 ms for the spectral MDCT part. FIG. 1A illustrates a prior art encoder and FIG. 1B illustrates a prior art decoder, both of which have corresponding delays associated with the MDCT core and the CELP core. Thus there is a need generally for alternative methods for coding audio signal bands that extend beyond the bandwidth of the core CELP codec in order to reduce algorithmic delay.
U.S. Pat. No. 5,127,054 assigned to Motorola Inc. describes regenerating missing bands of a subband coded speech signal by non-linearly processing known speech bands and then bandpass filtering the processed signal to derive a desired signal. The Motorola Patent processes a speech signal and thus requires the sequential filtering and processing. The Motorola Patent also employs a common coding method for all sub-bands.
The coding and reproducing of fine structure of missing bands by transposing and translating components from coded regions in the spectral domain is known generally and is sometimes referred to as Spectral Band Replication (SBR). In order for SBR processing to be employed where the speech codec operates at a bandwidth other than the input and output audio bandwidth, an analysis of the decoded speech would be required pursuant to ITU-T Recommendation G.729.1, Amendment 6 and ITU-T Recommendation G.718 Main Body and Amendment 2, resulting in relatively long algorithmic delay.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.